The objective of this study is to test the hypothesis that genetic and congenital factors are etiologically significant in certain cases of autism. Fifty autistic patients (25 per year), including twins (approximately 10 sets) and non-twin siblings (approximately 5 sets) will be identified for study by standardized diagnostic procedures. The following data will be obtained from all patients and their parents, siblings, and comparison cases when indicated. 1. Standardized family histories for neurological and psychiatric illness and developmental disabilities. 2. Behavioral Observation Scale (BOS) data. 3. Morphologic and dermatoglyphic measures. 4. Chromosome studies including gene markers and differential bandings. 5. Distribution of major histo-compatability complexes (MHC), Ia-like antigens, and quantitative and morphological studies of lymphocytes. 6. Family profiles of blood serotonin and platelet counts. 7. The prevalence of atopic diseases and evaluation of the T and B cell immunologic systems. Family, clinical, and laboratory data will be correlated and analyzed as the study progresses to insure that positive findings are substantiated and their relevance to the main hypothesis fully explored.